Apparatus for processing thermoplastic synthetic plastic material

ABSTRACT

An apparatus for processing thermoplastic synthetic plastic material to be recycled comprises a first receptacle ( 1 ) and a second receptacle ( 2 ) for the material to be processed. Within both receptacles ( 1, 2 ) tools ( 21 ) for mixing and heating the material circulate around a vertical axis each. The material reaches the second receptacle ( 2 ) which is connected to an evacuating means ( 9 ), from the first receptacle ( 1 ) through a connecting conduit ( 3 ). The first receptacle ( 1 ) has above it an intake opening ( 19 ) for the material to be processed, and the mouth of the connecting conduit ( 3 ) in the second receptacle ( 2 ) is disposed at a higher level than the tools ( 21 ) circulating in this receptacle ( 2 ). The material processed is carried off the second receptacle ( 2 ) through a discharge opening ( 49 ) by means of at least one screw ( 47 ), whereby this receptacle is vacuum-tightly closed. A sluice ( 6 ) is connected to the intake opening ( 19 ) of the first receptacle ( 1 ) that also is connected to an evacuating means ( 9 ). In both receptacles ( 1, 2 ) at least two tools ( 21 ) each circulate in different levels, disposed one upon the other, wherein the intake opening ( 19 ) in the first receptacle ( 1 ) is disposed higher than the uppermost tools ( 21 ) circulating in this receptacle ( 1 ). The discharge opening ( 49 ) of the second receptacle ( 2 ) is disposed at least substantially at the level of the lowermost tools ( 21 ) circulating in this receptacle ( 2 ). In each one of the two receptacles ( 1, 2 ) at least one temperature sensor ( 32 ) is provided for each plane of the circulating tools ( 21 ), which sensor is disposed higher than its associated plane.

BACKGROUND OF THE INVENTION

The invention relates to an apparatus for processing thermoplasticsynthetic plastic material that has to be recycled, comprising a firstand a second receptacle for the material to be processed, in whichreceptacles tools circulating around vertical axes are provided formixing and heating the material, wherein at least two tools eachcirculate in different levels one above the other and the materialreaches the second receptacle that is connected to an evacuating meansfrom the first receptacle through a connecting conduit, and wherein thefirst receptacle has above it an intake opening for the material to beprocessed, which intake opening is disposed at a higher level than theuppermost tools circulating in this receptacle and a discharge openingof the second receptacle is disposed at least substantially at the levelof the lowermost tools circulating in this receptacle, and wherein themouth of the connecting conduit in the second receptacle is disposed ata higher level than the tools circulating in this receptacle, andwherein the processed material is carried off the second receptaclethrough the discharge opening by means of at least one screw, wherebythis receptacle is vacuum-tightly closed, and wherein the firstreceptacle is also connected to an evacuating means.

Such an apparatus is known from WO 01/21372 A1.

Another apparatus having also two receptacles through which theprocessed material runs in series is known from EP 632759 B1.

As a rule, such apparatus work sufficiently if the material to beprocessed does not make specific demands, in particular on sensitivityagainst air access and/or superheat and/or if a reduction of themolecule chain length should be avoided.

SUMMARY OF THE INVENTION

The invention has as an object to improve an apparatus of the initiallydescribed kind so that also sensitive material, in particular PET(Polyethylene-Terephtalate), can be gently and continuously processedwithout the danger that this material is damaged by too intensive heatinfluence. In particular, for PET it is desired to avoid a reduction ofa IV-value (Intrinsic Viscosity), if possible even to increase thisvalue which is equivalent to an increase of the stability of theproducts processed from the recycled synthetic plastic material.

The invention solves this task so that a sluice is connected to theintake opening of the first receptacle and so that in both receptaclesthe tools are fixed to disc-shaped tool carriers disposed one above theother, wherein in each one of the two receptacles at least onetemperature sensor is provided for each level of the circulating tools,which sensor is disposed higher than the level associated to it. Therebythe synthetic plastic material to be recycled and supplied to theapparatus is continuously under vacuum from its introduction into thefirst receptacle until its conveying off the second receptacle by thescrew, so that damaging atmospheric influences are avoided and thematerial is processed similarly to within an autoclave. By the toolscirculating at different levels in each receptacle, for each receptaclea sufficient dwell-time of the material processed in the respectivereceptacle is obtained, which contributes substantially to obtaining ahomogenous working up of the material. In the first receptacle, thematerial supplied through the sluice reaches the uppermost circulatingtools from above and is taken along by these tools in a circulatingmotion around the receptacle axis in the form of a mixing cone. Thematerial processed by the tools disposed above in the first receptaclereaches the region below these tools and is taken along by the tooldisposed below and is circulated by this tool also in the form of amixing cone. This is repeated, the more tool levels are provided in therespective receptacle. Since to each one of these levels a temperaturesensor is associated which is disposed higher than the respective toollevel, by a suitable control of the values given by the temperaturesensors, overheating of the processed material and, therefore, a thermaldamage of it can be avoided everywhere.

From the first receptacle, the processed material reaches the secondreceptacle through the connecting conduit and is supplied within thisreceptacle also from above onto the tools circulating in thisreceptacle. Processing by these tools in the second receptacle takesplace in an analogous manner to that in the first receptacle; i.e. thematerial in the first instance is taken along by the uppermost tools andreaches the region of the tools disposed below. The thus completelyhomogenized material is finally pressed by the lowermost tools providedin the second receptacle into the exit opening of the second receptacleand is conveyed off by the screw connected to this opening. This screwacts vacuum-tightly, for example by compressing the material supplied toit so that the vacuum created within the second receptacle is maintainedalso when the processed material is conveyed off. At the inlet of thescrew there is a completely homogenized material that is crystallized inthe desired degree, although it is not at all thermally damaged.Surprisingly it has been shown that even an increase of the IV-value canbe obtained so that products with the desired high stability can beproduced from the so obtained recycled material.

By the disc-shaped tool carriers it is ensured that the processedmaterial can sink down from above to below only through the annular gapbetween the edge of this tool carrier and the inner wall of therespective receptacle. This enhances maintaining the desired dwell-timefor each single plastic particle within the respective receptacle.

According to a further embodiment of the invention the evacuating meansis equipped for creating different vacuum conditions in the tworeceptacles, and a transition sluice is disposed within the connectingconduit. Thereby there is the possibility to process within the tworeceptacles with different vacua, without the pressure difference beingimpaired by conveying the processed material from the first receptacleinto the second receptacle. Despite this vacuum sluice, a continuousprocessing manner is possible, since the sluice can be so operated thata continuous conveying off of the processed material from the firstreceptacle can be maintained and also a sufficient continuous supplyinto the second receptacle.

For maintaining the vacuum in the two receptacles a single vacuum pumpmay be sufficient. If different vacuum conditions in the two receptaclesare desired, it is, however, more favorable if the evacuating meanscomprises at least one vacuum pump for each receptacle. Further it issuitable within the spirit of the invention that a control means for thevacuum within the respective receptacle is connected to each receptacle,which control means adjustably controls the vacuum in the respectivereceptacle so that in each receptacle the always desired vacuum can beobtained and can be maintained.

According to a preferred embodiment of the invention, the disc edge ofat least one of these tool carriers is upwardly bent like a plate.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention can be seen from thedescription of an exemplary embodiment schematically shown in the singledrawing figure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The apparatus shown in the drawing comprises two receptacles 1, 2connected to each other by a connecting conduit 3. The supply of thethermoplastic synthetic plastic material to be processed, whichfrequently is PET (Polyethylene-Terephtalate), in particular in the formof milled material of bottles and bottle pre-forms, foils, fibers andthe like, therefore mostly in pre-comminuted form, takes place by aconveyor belt 4 into a hopper 5, to the outlet end of which a sluice 6is connected which can be designed as a vacuum sluice, whereby itsinterior 7 may be connected via a line 8 to an evacuating means 9. Thesluice 6 has above and below it two vacuum-tight closing gates 10, 11which can be shifted by double-acting cylinders 12 which suitably arehydraulically or pneumatically controlled. By this sluice 6 a sufficientvacuum can be maintained within the receptacle 1 also when the materialto be recycled is continuously introduced, whereby the interior of thereceptacle 1 is connected via a line 13 to a vacuum pump 14 of theevacuating means 9. This vacuum is continuously measured by a vacuummeasuring device 15, and the measured values are fed via a line 17 to acontrol means 16 which is connected via a line 18 to the evacuatingmeans 9 so that the evacuating means 9 provides continuously for thedesired vacuum within the receptacle 1.

The outlet end of the sluice 6 is connected to an inlet opening 19 inthe upper region or in the cover 20 of the receptacle 1 so that theintroduced synthetic plastic material to be recycled falls from aboveonto tools 21 mounted on a tool carrier 22 which by means of a verticalshaft 24 vacuum-tightly intersecting the receptacle bottom 23 is drivenfor rotation around the axis of the shaft 24 in the direction of thearrow 26 by a motor 25. Similar tools 21 are mounted on further toolcarriers 22 fixed spaced apart from each other to the shaft 24 below theuppermost tool carrier 22. The lowermost of these tool carriers 22 isdisposed closely above the bottom 23 of the receptacle 1 so that thetools 21 carried by it are disposed at the level of a discharge opening27 via which the housing 28 of a screw 29 is tangentially connected tothe receptacle 1. This discharge opening 27 constitutes simultaneouslythe inlet opening of the screw housing 28. By this, the circulatingtools 21 of the lowermost tool carrier 22 press the processed materialinto the volutions of the screw 29, by centrifugal action on one handand in the manner of a spatula on the other hand, if the tools 21 arecorrespondingly disposed at the tool carrier 22.

The tools 21 of each one of the tool carriers 22 make the syntheticplastic material taken along by them circulate around the axis of thereceptacle 1, whereby the material rises in the form of a mixing cone30. Spaced apart above the tool carrier 22 a temperature sensor 32 isdisposed in the region of the mixing cone at the side wall 31 of thereceptacle 1. Each temperature sensor 32 is connected via a line 33 to acontrol means 34. This control means 34 controls via a line 40 therotational speed of the motor 25 driving the tools 21 to theircirculating motion so that any overheating of the material processedwithin the receptacle 1 is avoided.

The screw 29 driven by a motor 35 conveys the material processed withinthe receptacle 1 into the connecting conduit 3 which merges via an inletopening 37 into the second receptacle 2, which opening is disposed inthe cover 36 or in the upper region of the second receptacle 2. Withinthis receptacle 2 circulating tools 21 are mounted on tool carriers 22in a similar manner as has been described in connection with thereceptacle 1, which tool carriers are disposed below the inlet opening37 and are driven for rotation via a shaft 38 by a motor 39. The speedof this motor 39 is also controlled by a control means 41 via a line 42so that the temperature of the synthetic plastic material processedwithin the receptacle 2 sensed by temperature sensors 32 does not exceedpredetermined set temperature values.

The tools 21 can also be driven from above. In such a case the motor 25or 39, respectively, is disposed above the receptacle 1 or 2,respectively, and the shaft 24 or 38, respectively, intersectsvacuum-tightly the cover 20 or 36, respectively.

The receptacle 2 can also be evacuated, and for this it is connected viaa line 43 to a vacuum pump 44 of the evacuating means 9. Bothreceptacles 1, 2 can be evacuated by means of the same vacuum pump. Inorder to increase the versatility of the plant, however, it is to bepreferred to provide separate vacuum pumps 14, 44 for the tworeceptacles 1, 2, because in such a manner different vacuum conditionscan be obtained and maintained within the two receptacles 1, 2 in asimple manner. The pressure within the receptacle 2 is measured by meansof a vacuum measuring device 45 and is indicated via a control means 46to the evacuating means 9 so that there are always the desired vacuumconditions within the receptacle 2, in an analogous manner as this iswithin the receptacle 1.

From the receptacle 2, the processed synthetic plastic material isdischarged by means of a screw 47, the housing 48 of which is connectedvia a discharge opening 49 to the receptacle 2 in a similar manner ashas been described for the receptacle 1. This discharge opening 49 isdisposed at the level of the tools 21 carried by the lowermost toolcarrier 22 of the receptacle 2, so that filling of the screw 47 takesplace in a similar manner as filling of the screw 29. The core diameterof the screw 47 increases in the direction of conveyance (arrow 50), sothat the material supplied to the screw through the discharge opening 49is at first compressed. Thereby, the screw 47 acts together with itshousing 48 as a vacuum-tight closure of the receptacle 2. Following thiszone of compression, the core diameter of the screw 47 decreases, sothat a relaxing zone 51 for the material conveyed by the screw 47 iscreated. In this relaxing zone 51 at least one de-gassing opening 52 mayintersect the screw housing 48. To the openings 52 lines 53 forconveying off the gases are connected. This conveying off of gas can beassisted by a vacuum pump 54 of the evacuating means 9. The vacuumcreated by the pump 54 can be controlled by means of a vacuum measuringdevice 55.

If desired, the vacuum pumps 14, 44, 54 can be distributed to differentevacuating means 9. However, as a rule, the vacuum-creating members willbe combined to one single evacuating means 9.

If different vacuum conditions are to be maintained within the tworeceptacles 1 and 2, and the screw 29 and its housing 28 do notconstitute a vacuum-tight closure between the two receptacles 1, 2, itis suitable to provide a transfer sluice 56 in the connection conduit 3,the sluice chamber 57 of which is confined by two gates 58, 59 moved bycylinders 60 in an analogous manner as is the case for the sluice 6.Further, also the sluice chamber 57 of the transfer sluice 56 isconnected via a line 61 to the evacuating means 9.

The screw 47 is driven by a motor 64 and suitably constitutes a memberof an extruder 62 conveying the material supplied by the screw 47 in theform of strands to a granulating apparatus. This extruder 62 may be adouble screw extruder. However, it is also possible that the screw 47acts as a mere conveying screw only and feeds the material conveyed byit to further processing, for example (after plasticizing) to a molddevice.

It is suitable to insert dust separators 66 into the lines 17, 43 viawhich evacuating of the receptacles 1, 2 takes place, which separatorsfilter the dust from the air sucked off which is created by rubbing offwhen the synthetic plastic mass is processed within the receptacle 1 or2, respectively.

By forming tool carriers 22 as discs having a circular cross-section, anannular gap 63 is created between the edge of each disc and the sidewall 31 of the respective receptacle 1 or 2. Only through this annulargap the material can bypass the respective disc-shaped tool carrier 22from above to below, which substantially contributes to ensure asufficient dwell-time for each plastics particle within the respectivereceptacle 1 or 2. The arrangement of tool carriers 22 contributes tothis, because each tool carrier with its tools 21 again whirls up theplastics material supplied to it from above in the form of the mixingcone 30. The number of tool carriers 22 disposed one above the otherwithin the respective receptacle 1 or 2 depends on the field ofoperation considered. However, within each receptacle 1, 2 at least twotool carriers 22 should be provided one above the other.

It is suitable to bend or angle the disc edge 65 to above like a plate,because this favors creating an upwardly extending mixing cone andcounteracts an early sinking down of the plastics particle through thegap existing between the disc edge and the receptacle wall.

If the apparatus is designed for processing already pre-comminutedplastics material, for example milled PET-bottles, then the tools 21need not act in a comminuting manner, and they can be mere mixing toolsthat act to heat the processed material so that the kinetic powersupplied to the tools mostly changes into heat power transferred intothe material processed. However, if comminuting of the plastics materialto be processed is required, then it is suitable to provide the tools 21with cutting edges, in the manner of knives, whereby these cutting edgesare so disposed that a drawing cut is obtained when the tools circulate(arrows 26).

As already mentioned, the temperature sensors 32 in the two receptacles1, 2 are disposed higher than the respective tool carrier or the tools21, respectively, in the mixing cone region of which the respectivesensor 32 is disposed. In many cases a favorable height for disposal ofthe sensors is in the region in which the respective mixing cone 30leaves the wall 31 of the receptacle 1 or 2, respectively.

The screw housings 28 or 48, respectively, must not be connectedtangentially to the respective receptacle 1 or 2; also a radial disposalor a disposal in the manner of a secant with respect to the receptacleis possible. However, the tangential disposal has the advantage that themotor 35 or 64 driving the screw 29 or 47 can be disposed at the onefront end of the respective screw, and the discharge end of the screw atthe other front end of the respective screw housing. This avoids alateral deviation of the material conveyed by the screw.

1. Apparatus for processing thermoplastic synthetic material that has tobe recycled, comprising a first and a second receptacle for the materialto be processed, in which receptacles tools circulating around verticalaxes are provided for mixing and heating the material, wherein at leasttwo tools each circulate in different levels one above the other, andthe material reaches the second receptacle that is connected to anevacuating means from the first receptacle through a connecting conduit,and wherein the first receptacle has above an intake opening for thematerial to be processed, which opening is disposed at a higher levelthan highermost tools circulating within this receptacle, and adischarge opening of the second receptacle is disposed at leastsubstantially at the level of lowermost tools circulating in thisreceptacle, and a mouth of the connecting conduit in the secondreceptacle is disposed at a higher level than the tools circulating inthis receptacle, and wherein the processed material is carried off thesecond receptacle through the discharge opening by means of at least onescrew, whereby this receptacle is vacuum-tightly closed, and the firstreceptacle is also connected to an evacuating means, wherein to theintake opening of the first receptacle a sluice is connected and whereinin both receptacles the tools are mounted on disc-shaped tool carriersdisposed one above the other, and wherein in each one of the tworeceptacles at least one temperature sensor is provided for each levelof the circulating tools, which sensor is disposed higher than the levelassociated to it.
 2. Apparatus according to claim 1, wherein theevacuating means is equipped for creating different vacuum conditions inthe two receptacles and a transfer sluice is disposed in the connectingconduit.
 3. Apparatus according to claim 2, wherein the evacuating meanscomprises at least one vacuum pump for each one of the receptacles. 4.Apparatus according to claim 1, wherein a control means for the vacuumwithin the respective receptacle is connected to each one of thereceptacles, which control means adjustably controls the vacuum in therespective receptacle.
 5. Apparatus according to claim 1, wherein on atleast one of the tool carriers the disc edge is upwardly bent like aplate.
 6. Apparatus according to claim 1, wherein the temperaturesensors are connected to means for controlling the circulation of thetools.
 7. Apparatus according to claim 1, wherein each one of theevacuating means comprises a dust separator.
 8. Apparatus according toclaim 1, wherein the screw constitutes a member of an extruder. 9.Apparatus according to claim 8, wherein a double screw extruder isconnected to the discharge opening.
 10. Apparatus according to claim 1,wherein to the discharge opening a housing of the screw is connected,which housing comprises at least one de-gassing opening.
 11. Apparatusaccording to claim 1, wherein each temperature sensor is disposed withinthe receptacle at least substantially at a level that is in the regionin which a mixing cone leaves the side wall of the receptacle. 12.Apparatus according to claim 1, wherein the sluice is a vacuum sluice.13. Apparatus according to claim 1 wherein the mouth of the connectingconduit in the second receptacle is disposed at a higher level than anuppermost tool in the second receptacle.
 14. Apparatus according toclaim 10 including a vacuum pump connected to the degassing opening.